UV irradiation resistance-associated gene (UVRAG) has been implicated in diverse cellular processes including autophagy, endocytic trafficking and chromosome maintenance. UVRAG was first identified from a cDNA library screening for its ability to complement partially the ultraviolet sensitivity of a xeroderma pigmentosum cell line (Perelman et al., 1997). UVRAG was recently found to be a key regulator of the Class III Phosphotidylinositol 3-Kinase (PI3K) complex, a critical component of the molecular machinery of autophagy consisting of the scaffolding protein Beclin 1 and the lipid kinase VPS34 as core members. Through potent and specific interaction with Beclin 1, UVRAG can lead to the formation of UVRAG-containing Beclin 1-VPS34 complex with enhanced lipid kinase activity to direct VPS34-related cellular processes such as autophagy (Liang et al., 2006; Liang et al., 2007). UVRAG has also been found to associate with Class C Vps complex and coordinate endocytic trafficking (Liang et al., 2008a; Liang et al., 2008b). Furthermore, UVRAG plays a role in maintaining structural integrity and proper segregation of chromosomes through its interactions with centrosome protein CEP63 and DNA-PK that is involved in homologous end joining (Zhao et al., 2012).
UVRAG contains two well predicted functional domains based on sequence alignment. The N-terminal C2 domain is regarded to associate with membrane and be involved in autophagy and endosomal trafficking (Liang et al., 2006). The coiled coil (CC) domain is critical for binding to Beclin 1, the essential autophagy scaffolding protein, to form the autophagy-promoting UVRAG-containing Beclin 1-VPS34 complex (Liang et al., 2006). In addition to these two domains, the N-terminal proline-rich sequence of UVRAG interacts with the SH3 domain of Bif-1 and probably enables Bif-1 to promote autophagosome formation through its membrane-curving BAR domain (Takahashi et al., 2007; Takahashi et al., 2009). The region between the coiled coil domain and the C-terminal PEST-like sequence is involved in interaction with Class C Vps complex, CEP63 and DNA-PK (Liang et al., 2008a; Zhao et al., 2012).
No structural information at atomic resolution is currently available regarding UVRAG, and the molecular mechanism of how the individual functional domains of UVRAG associate with their respective binding partners to regulate diverse cellular processes of autophagy, endocytic trafficking and chromosomal segregation is not well understood.
The interaction between Beclin 1 and two central autophagy regulators Atg14L and UVRAG is mediated through their respective coiled coil domains (Liang et al., 2006; Matsunaga et al., 2009; Zhong et al., 2009). The structure of the Beclin 1 coiled coil domain was determined previously, which forms a metastable antiparallel coiled coil structure due to several charged or polar residues that destabilize an otherwise hydrophobic dimer interface (Li et al., 2012a). This metastability is found to be important for Beclin 1's interaction with Atg14L or UVRAG because it enables the homodimeric Beclin 1 to readily dissociate and form heterodimeric assembly with Atg14L and UVRAG (Li et al., 2012a). Mutations within the Beclin 1 coiled coil domain that render it monomeric retains its binding to Atg14L or UVRAG and facilitates normal autophagy induction; while mutations that stabilize the Beclin 1 homodimer weaken or abolish its interaction with Atg14L and lead to impaired autophagosome formation (Li et al., 2012a; Li et al., 2012b).
The mammalian Class III phosphatidylinositol 3-kinase (PI3KC3) complex, also termed the Beclin 1-Vps34 complex, is a dynamic multi-protein assembly that plays critical roles in membrane-mediated intracellular transportation processes such as autophagy, endocytic trafficking and phagocytosis. Core members of this complex include the lipid kinase Vps34 that serves as the major producer of phosphatidylinositol 3-phosphate (PI3P) lipids; a serine/threonine kinase Vps15 stably associated with Vps34, the scaffolding molecule Beclin 1 and either Atg14L or UVRAG as the Beclin 1-binding partner. The Atg14L-containing form is termed Beclin 1-Atg14L complex and mainly involved in early-stage autophagy induction because Atg14L is responsible for directing Beclin 1-Atg14L complex to ER sites to promote autophagosome biogenesis. The UVRAG-containing form, on the other hand, is termed Beclin 1-UVRAG complex and plays critical roles in late-stage autophagy execution and degradative endocytic trafficking. In addition to these core molecules, many regulators such as Ambra1, Bcl-2, NRBF2 and Rubicon can associate with the Beclin 1-Vps34 complex in dynamic and context-dependent manner to exert modulatory effect on the Vps34 kinase activity. The molecular mechanism of such regulation, particularly whether these diverse molecules share a common theme of modulating the structural and thus biochemical properties of the Beclin 1-Vps34 complex, is not well understood.